Researchers at MIT have developed a metal-free electrode using conductive polymers. The electrode is flexible and strong enough for long-term implantation in the body. The device is intended as an advanced replacement for rigid metal electrodes that can cause tissue damage and scarring over the long term, leading to device failure. The new technology required quite a bit of refinement to achieve the correct properties of flexibility, strength, and electrical conductivity. The electrode material can be printed using a 3D printer, meaning that the researchers can easily create a vast array of complex geometries and shapes to meet the needs of a wide variety of medical technologies.
Implantable technologies are advancing to behave and feel more like human tissues, compared to rigid mechanical devices. There are numerous advantages to this – flexible implants are less likely to cause damage in soft tissues and are also less likely to cause scarring and inflammation. The foreign body response and scar tissue can lead to implant failure, and if long-term implantable devices are to emerge then developing high-end electrodes that allow them to interact with tissues for many years will be necessary.
This latest technology is a step in the right direction. It is a completely metal-free electrode, made using conductive polymers. “This material operates the same as metal electrodes but is made from gels that are similar to our bodies, and with similar water content,” said Hyunwoo Yuk, a researcher involved in the study. “It’s like an artificial tissue or nerve.”
The electrode was challenging to create, as polymers are typically insulative rather than conductive. While conductive polymers have been identified, crafting them into a flexible gel-like electrode was no easy feat, and required the researchers to balance conductive properties with mechanical limitations.
“In gel materials, the electrical and mechanical properties always fight each other,” said Yuk. “If you improve a gel’s electrical properties, you have to sacrifice mechanical properties, and vice versa. But in reality, we need both: A material should be conductive, and also stretchy and robust. That was the true challenge and the reason why people could not make conductive polymers into reliable devices entirely made out of gel.”
Their solution was to combine conductive polymers with other hydrogel components that can provide the required mechanical properties. The key to achieving this was to induce phase separation, where the materials slightly repel each other.
“Imagine we are making electrical and mechanical spaghetti,” said Xuanhe Zhao, another researcher involved in the study. “The electrical spaghetti is the conductive polymer, which can now transmit electricity across the material because it is continuous. And the mechanical spaghetti is the hydrogel, which can transmit mechanical forces and be tough and stretchy because it is also continuous.”
Study in journal Nature Materials: 3D printable high-performance conducting polymer hydrogel for all-hydrogel bioelectronic interfaces